As coal is a heterogeneous mixture of organic and inorganic constituents, the process of solvolysis of coal varies with its constituents, maturity, and structural characteristics. Since the mineral matter (non-combustible) in coals available in specific geographical location, is very finely disseminated in the organic mass, it is quite difficult to remove this non-combustible mineral matter by conventional physical coal washing techniques. Presence of high percentage of near gravity material in coal makes the scope of gravity process limited. It is known that chemical benefication originates from the limitations of physical beneficiation processes. Broadly, chemical beneficiation is possible by chemical leaching of mineral matter present in coal or, dissolving organic matter of coal in various organic solvents. This indicates that chemical treatment could be one of the solutions to overcome the limitations of physical benefication methods. Prior art teaches chemical beneficiation techniques that employ highly, corrosive chemicals (mostly acids and alkalis). Recovery or regeneration of these chemicals is very important to make this technology viable. A parallel approach towards lowering the ash-content could be recovery of the premium organic matter from coal by solvent refining. Most of the prior art disclose that chemical leaching is basically adapted to produce ultra clean coal or super clean coal with ash content less than 0.2% for various high tech end uses. However, such conventional solvent refining processes do not serve the objective of low ash coal requirement of steel industries because of mainly low recovery which makes the process uneconomic especially when such an ultra clean coal is not absolutely desired at the cost of lowering the yields. Additionally, the operating cost of said prior art process is high because of high cost of solvents and energy requirement in the process. In prior art process, the extraction is being done at boiling point of the solvent mixture making it difficult to recover the solvent from clean coal and reject. Thus, there is a need to propose a process of washing clean coal and reject to recover the remaining solvents. Also, there is a need to develop a process of extraction of coal at a temperature lower than the boiling point of the solvent mix.
By way of reference, the inventors observed that Indian patent application numbers 1292/KOL/06, 1088/KOL/07, 1336/KOL/2008, 950/KOL/09, 1194/KOL/09, 611/KOL/09, 1581/KOL/08 are herein incorporated.